FIELD OF THE INVENTION
The present invention relates a flyback circuit, and particularly a synchronous rectifier flyback circuit for zero voltage switching {ZVS} in continuous mode {CCM} and discontinuous mode {DCM}, which circuit minimized loss generated when electrifying parasitic diode of MOS transistor {MOSFET} that is secondary side switch of synchronous rectifier and which also enabled ZVS in whole range of discontinuous mode {DCM}.
Recently we are under state of continual inevitable request of system specification augmentation such as multimedia system construction, acceleration of CPU to high speed, memory increase according to worldwide trend that notebook PC becomes day by day smaller, lighter, slimmer and pursuing high functionalization.
And as capacity for each resource of each system specification is increasing, though now AC adapter for notebook PC uses 4550 watts(W), gradually need is rising up for making high capacity of 60 W, 75 W, 80 W or more, for making it a micro slip easy and simple of carrying and handling, and for high efficiency.
Particularly the reason to make AC adapter in high efficiency is that higher efficiency means lower internal electricity loss, which means lower internal heat generation so that miniaturization be feasible.
Whence the most typical methods used now for AC adapter are flyback circuit method and resonance type method. Among them, flyback circuit method has disadvantage of large electricity loss because this method performs hard switching where there is large intersection of turn-on current (Ids) and turn-off voltage (Vds) of MOSFET that is semiconductor element while resonance type method is effective method for miniaturization and making its weight lighter but contains disadvantage that it has bad controllability because it makes voltage and current in sinusoidal shape and that the voltage and electric current stresses are great in switching element.
Therefore nowadays attention is led to synchronous rectification method that uses a synchronous rectifier {SR} because it has high efficiency. SR uses MOSFET instead of output diode and, although there occurs R.sub.ds(on) loss (I.sub.F.sup.2 * R.sub.ds(on)) when SR is electrified, R.sub.ds(on) is very small as much as 0.020.about.0.025.OMEGA. TO generate only small loss so that SR is highly effective in efficiency enhancement.
Appended FIG. 1 shows an example of flyback circuit applying the existing synchronous rectifier among conventional AC adapter formations applying the synchronous rectification method as above.
As shown there, it is composed of transformer (T) inducing the primary side energy to secondary side, switch (SW) which switches primary voltage of said transformer (T), and synchronous rectifier (SR) that rectifies secondary voltage of said transformer (T).
Flyback circuit formed as above, applying the conventional SR, is operated in continuous mode {CCM} and discontinuous mode {DCM} which will be respectively explained below.
At first, in case of CCM operation, if switch (SW) gate voltage represented as 2a in FIG. 2 is controlled, primary side current (ipri) of transformer (T) increases linearly in the ON operation section of said switch (SW) as shown by 2b in FIG. 2.
Whence energy is accumulated at primary side coil (Lm) of said transformer (T) in the ON operation section of said switch (SW) and then the said transformer (T) polarity is converted at said switch (SW) turn-off time point so that electric current (isec) flows to secondary side of said transformer (T) in manner of electric current wave shape represented by 2c in appended FIG. 2. This may be known as to operate in CCM flowing in continuous manner without section where the flow is null
Voltage wave shape signified by 2d in appended FIG. 2 is the said switch (SW) drain source voltage wave shape while voltage wave shape signified by 2e depicts secondary voltage of said transformer (T).
In appended FIG. 2, T.sub.SR is the ON section of MOSFET (SR) used as synchronous ammeter while D.sub.SR is electrification section of parasitic diode (D2) of said MOSFET (SR). And because the electrification section (D.sub.SR) of said parasitic diode (D2) is also loss, efficiency will be ideally maximum without loss if V.sup.SR.sub.GS is turned on at just V.sup.SW.sub.GS turnoff time point and V.sup.SW.sub.GS is turned on at just V.sup.SR.sub.GS turnoff time point.
In other words, delay time of T.sup.ON.sub.D and T.sup.OFF.sub.D are needed to exist between V.sup.SW.sub.GS turnoff and V.sub.GS.sup.SR turnon and between V.sub.GS.sup.SR turnoff and V.sup.SW.sub.GS turnon respectively because V.sup.SW.sub.GS ON section is. energy storage period in Lm and output condenser (C3) discharge will arise if V.sup.SW.sub.GS is turned on in V.sup.SR.sub.GS ON section.
FIG. 3 is wave shape example diagram for discontinuous mode operation case of flyback circuit such as in FIG. 2. In FIG. 3, part signified as 3a is switch (SW) gate drive voltage while 3b and 3c are primary and secondary electric currents of transformer (T) respectively. And 3d is drain source voltage of the switch (SW).
Now discontinuous mode operation case is explained below.
Because inductance is small in discontinuous mode, the energy stored in primary side (Lm) of transformer signified by reference symbol 3a in FIG. 3 during V.sup.SW.sub.GS turnon interval is completely consumed during V.sup.SW.sub.GS turnoff before V.sup.SW.sub.GS is turned on, so that secondary current I.sub.SEC becomes "0".
Then after said secondary side current (isec) of said transformer (T) became "0", T.sub.DCM section from time point signified by t3 to time point signified by t4 is LC resonance section owing to sum of parasitic capacitance (C1) existing in said switch (SW) and parasitic capacitance (C2) existing in said synchronous rectifier (SR) (C.sub.eq =C.sup.SW.sub.OSS C.sup.SR.sub.OSS /n.sup.2) which means total capacitance and said transformer (T) leakage inductance (L.sub.lk). This n2 TDCM section varies by input voltage and load.